fm10k: update comment regarding reserved bits check

The original comment may be read incorrectly as referring to checking
the *entire* length is zero. However, it merely checks only the reserved
bits of both length and reserved in a small amount of code. Update the
comment to indicate this is a clever trick and clearly spell out that it
only checks the reserve bits.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Krishneil Singh <Krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/fm10k/fm10k_vf.c

@@ -188,7 +188,7 @@ static s32 fm10k_update_vlan_vf(struct fm10k_hw *hw, u32 vid, u8 vsi, bool set)
 	if (vsi)
 		return FM10K_ERR_PARAM;
 
-	/* verify upper 4 bits of vid and length are 0 */
+	/* clever trick to verify reserved bits in both vid and length */
 	if ((vid << 16 | vid) >> 28)
 		return FM10K_ERR_PARAM;